In the process of making paper, it is important to determine and keep close track of the moisture content of the wood being processed as the moisture content affects pulp yield during the pulping and paper making processes and thus can greatly influence the cost of pulping and paper making. For example, when wood is purchased in lots, it is generally purchased based on dry weight of the wood. Therefore, it is important to know the moisture content of the wood in order to more accurately calculate the dry weight of the wood. In addition, it is important to determine the moisture content of the wood chips being fed to the digesters along a pulping line to enable greater control of the cooking liquor being charged into the digesters and to enable greater control of the liquor/wood and chemical to wood ratios in the digesters to better control and thus lower the operating costs for the digester line.
Such moisture determinations can be made in "real-time" as the wood chips are fed to the digester using on line analyzers, such as the Micromoist Analyzer manufactured by Berthold. Such analyzers, however, must be precisely calibrated to insure accuracy in their measurements of the moisture content of the wood chips being fed to the digesters. In addition, for other applications in which the moisture content of wood and/or wood chips is determined, the processes for determining the wood chip moisture content must be carried out as efficiently and quickly as possible, preferably on site at a paper mill, to minimize the potential for variations in the calculated moisture content and actual moisture content that could occur over extended time delays due to changes in the moisture of the sampled material resulting from condensation of moisture in the sample container, diffusion of moisture out of the container, and possible stratification of moisture within the large samples required by the current methods.
There are currently two primary or standard methods for determining or calculating moisture content of wood/wood chips for use in calibrating the micromoist analyzers or meters. These include the TAPPI T208 OM-94 standard method for analyzing "Moisture in Wood, Pulp, Paper and Paperboard by Toluene Distillation", and oven drying of wood chip samples. In the first method by which moisture content in wood, pulp and paper is determined by toluene distillation, toluene, a hazardous/flammable liquid, is used to distill water out of a quantity or sample of wood chips. Since water is not miscible with toluene, a layer of water generally is formed in a receiving vessel after application of the toluene, which water can then be measured to determine the moisture content. However, this method has certain serious drawbacks or problems including toluene being a hazardous carcinogenic and highly flammable material, the use of which requires a large distillation apparatus and significant fume hood space for handling hazardous material, both of which is expensive and space intensive. This method further is very labor intensive as it requires close monitoring of the distillation continuously over a period of five or more hours needed to insure that all the water is extracted from the samples.
The second method, the oven drying method is much simpler, and typically does not require special equipment for handling hazardous materials such as fume hood space, etc. Instead, this method involves drying a measured quantity of wood chips in an oven for approximately 24 hours, taking measurements from the samples both before and after drying to determine amount of moisture within the sample. A significant problem with such a method is that samples are added and removed from the oven sporadically over a 24-hour period, a moisture free environment could not be maintained which resulted in large variations in the subsets of the samples. In addition, temperature and air circulation within the oven also affects the drying rates of the samples so that some samples dried faster than others, creating variations. Further, when wood is oven dried, all the material within the wood in addition to the moisture or water in the wood that is volatile at the oven temperatures tends to evaporate, thus causing overestimation of the amount of water in the wood chips.
In addition, another significant factor with using both of these processes is that to ensure significant accuracy in developing a calibration curve for calibrating micromoist analyzers, approximately 30 moisture determinations spanning the expected range are necessary. Each of the moisture determinations generally are conducted with an average subset of approximately five samples of wood chips, such that upwards of 150 separate samples must be analyzed in order to get a substantially complete and accurate calibration curve. With such a large number of samples typically needed to insure an accurate calibration curve, the time and apparatus required for conducting these tests is significantly extensive, for example, requiring large fume hood space or large ovens and close monitoring, thus significantly increasing the cost of performing these methods. Further, given the necessity of performing such determinations as rapidly as possible to minimize the potential for variations or changes in the moisture content of the wood after sampling, i.e., from condensation of moisture from the sample to the container, diffusion of moisture out of the sample container, and possible stratification of moisture within the large samples required by the current methods, it is not practical to ship large samples of the wood chips being processed to outside laboratories due to the time and expense required for testing such a large number of samples.
Accordingly, it can be seen that a need exists for a method of determining moisture content in wood for use in a paper making process that is inexpensive to perform and which does not require extensive processing equipment and which further can be performed quickly and accurately on site at a paper mill.